1. Field of the Invention
The present invention relates to a method and apparatus for determining the momental wavelength of the spectrum of light. In addition, the present invention relates to an optical amplifier and an optical communication system which incorporate the method and apparatus to adjust gain tilt in accordance with the determined momental wavelength.
2. Description of the Related Art
Optical communication systems typically use wavelength-division multiplexing to increase transmission capacity. More specifically, a plurality of signal lights each having a different wavelength are multiplexed together into a wavelength division multiplexed (WDM) signal light. The WDM signal light is transmitted over a transmission line, and then demultiplexed at the other end of the transmission line so that the individual signal lights can be individually received. The transmission line is usually a single optical fiber.
An optical amplifier is typically used in such an optical communication system to amplify the WDM signal light, since an optical amplifier has a relatively broad band. More specifically, the broad band of the optical amplifier allows each of the individual signal lights in the WDM signal light to be amplified.
Generally, an optical amplifier includes an optical amplifying medium, such as an erbium-doped fiber (EDF). The WDM signal light travels through the optical amplifying medium. The optical amplifier also includes a light source, such as a laser diode, which provides pump light to the optical amplifying medium. The pump light causes the WDM signal light to be amplified as the light signal travels through the optical amplifying medium. Repeaters, each have an optical amplifier, are typically interposed into the transmission line to transmit a WDM signal light over a long distance.
Moreover, the gain of an optical amplifier is dependant on the wavelength of the amplified signal. This dependance is defined as the "gain tilt" of the optical amplifier. Therefore, when a WDM signal light is amplified by the optical amplifier, each of the individual signal lights multiplexed together may be amplified with a different gain. This gain tilt of the optical amplifier must be considered when using an optical amplifier to amplify a WDM signal light.
Therefore, the gain tilt of an optical amplifier should be monitored or controlled when using the optical amplifier in an optical communication system which used wavelength division multiplexing. A conventional spectrum analyzer can be used to monitor the gain tilt of an optical amplifier. However, a spectrum is expensive and relatively large device.